Image forming apparatuses such as laser printers have a developer cartridge that generally includes a housing, an agitator, a development roller and a supply roller, which are rotatably disposed in the housing. The agitator agitates toner. The development roller is configured to bear toner on its surface. The supply roller supplies toner to the development roller. The development roller of the developer cartridge is disposed to confront a photosensitive drum. The toner borne on the development roller is transferred to the electrostatic latent image formed on the photosensitive drum. The latent image is thereby developed.
As disclosed in U.S. Pat. No. 6,823,160 (corresponding to Japanese Patent Application Publication No. 2003-295614), the shaft of the development roller protrudes outside from the housing of the developer cartridge. A fixed gear is mounted on the part of the shaft that protrudes from the housing. The fixed gear therefore rotates when the development roller is rotated. The fixed gear is engaged with a plurality of gears, which are held in a protection cover and prevented from coming out of engagement. These gears transmit a driving force from the main body of the image forming apparatus that includes developer cartridge. Thus, the development roller is rotated.